Geothermal steam ejected from a production well at a geothermal generation facility and supplied to the steam turbine includes various mineral components and gas components. Situations leading to trouble in a geothermal generation output can increase, such as the power of the turbine decreasing when these mineral components adhere to a turbine blade, or the pressure of a steam condenser increasing when non-condensable gas components included in the geothermal steam. For this reason, Japanese Patent Publication No. JP 2002-250271 A (herein “Document 1”) proposes a countermeasure for quantitative display and understanding of a scale coating condition resulting over time during normal operation and a scale removal work or clearing work of the turbine blade. In this Patent Document 1, the scale coating condition is monitored by monitoring, while the turbine is operated, decrease over time in nozzle output pressure, which results from increase in amount of adhered scales and ejection on a nozzle plate of the steam turbine through which natural steam ejected from underground is guided, and resulting in decrease in the turbine output.